It is known to provide a polymer web which can be used for the sealing of roof surfaces and other surfaces of a structure against the penetration of water or moisture which has an upper sealing layer of black polymer bitumen in addition to a support of a woven or nonwoven fabric, fleece or fiber and a sealing surface on the underside.
To protect the upper surface, it is known to provide a broken rock or rock-fragment layer usually of schist or shale, the rockfragment layer or broken-rock covering tending to penetrate into at least the upper sealing layer and in many cases to weaken it.
A further disadvantage of this surface protection is the tendency of the rock-fragment layer to crack, spall or burst at the high temperatures applied in welding the web edges together.
Such high temperatures are necessary to weld edges of the strip together and assures bonding of the strips in an impervious manner.
The bursting of the rock-fragment layer, especially in overlap regions at which the seams are formed, may interfere with the formation of an effective seal.
In practice it is not possible to avoid the presence of the rock-fragment layer in the overlapping regions and the very presence of the rock-fragment layer at the interface also reduces the reliability of the seal or prevents the formation of a seal altogether.
Another drawback of earlier systems is the tendency, after the rock-fragment layer or shale layer has worn away by weathering of the upper sealing layer to undergo photochemical degradation, generally as a result of the sun's rays. This degradation sharply reduces the life of the roof covering.
It has been proposed to prevent this or to retard the photochemical degradation by providing a comparatively expensive gravel covering which is not possible in many cases on purely static grounds.